The invention relates to a process for supplying energy to a low-pressure UV irradiation lamp in accordance with the preamble of claim 1 and a ballast for supplying energy to a low-pressure UV irradiation lamp in accordance with the preamble of claim 7.
Methods for disinfecting water by means of UV light are making use of increasingly powerful low-pressure irradiation lamps. Requirements in terms of effectiveness and adjustability are very high.
Whereas gas discharge lamps for purposes of lighting function mostly with simple ballasts containing passive components and receive their energy directly from the low-voltage mains at the normal mains frequency of 50 to 60 Hz, powerful low-pressure UV irradiation lamps for water disinfection are operated with electronic ballasts and at mains frequencies  greater than 20 KHz. Operating at a considerably higher frequency than the normal mains frequency has the advantage that passive components used, such as inductors and capacitors, can be smaller in terms of size and weight. In addition, the ionization of the gas discharge arc is not lost after zero crossover of the radiation current when the polarity changes, whereas at the normal mains frequency, ionization is interrupted by ion recombination at every zero crossover of the radiation current, so that the low-pressure UV irradiation lamp has to be restarted after every zero crossover.
On the other hand, the disadvantages of operating at frequencies  greater than 20 KHz include the presence of perturbing radiation and line losses over longer line distances between the ballast and the low-pressure UV irradiation lamps. Both of these disadvantages are also particularly significant in applications related to the water disinfection, for as the power of the UV lamp is increased, so too does the perturbing radiation. Moreover, specifically in water treatment applications, whole batteries of low-pressure UV irradiation lamps are used in a limited space. If it is not possible to deploy the ballasts in this space as well, appropriate provision must be made for long energy supply lines.
In the case of gas discharge lamps for lighting purposes, it is known from DE 36 07 109 C1, DE 44 01 630 A1 or DE 196 42 947 A1 to avoid strobe effects and flickering at the mains frequency, and to reduce alternating electromagnetic fields by the use of direct current. However, since operation exclusively with direct current leads to electrophoretic effects that cause the contents of the lamp to be deposited on the interior surface of the lamp glass and the electrodes, and a corresponding loss of light output, the polarity in gas discharge lamps is reversed from time to time. Intervals of between 15 and 30 minutes are indicated for this.
It has been demonstrated that when these measures used in gas discharge lamps for lighting are applied to low-pressure UV irradiation lamps, both the operating life and the irradiating performance of such irradiation lamps are severely degraded.
The object of the present invention is to simplify the energy supply required for operating low-pressure UV irradiation lamps, to increase the UV light output and to improve efficiency without shortening the operating life.
This object is solved in a process according to the preamble of claim 1, by the characterizing portion of that claim, and in a ballast according to the preamble of claim 7, and the characterizing portion of that claim.
Improvements and advantageous configurations of the invention are described in the subordinate claims.
A partial solution to the process according to invention consists in known manner of operating the low-pressure UV irradiation lamp with direct voltage or direct current. This eliminates all the disadvantages associated with and alternating voltage or alternating current energy supply, that is to say for mains frequency energy supply the constant restriking of the gas discharge arc at the mains frequency with the consequentially increased electrode wear, and for high-frequency energy supply  greater than 20 KHz, perturbing radiation and short lengths of energy supply lines or line losses. This also prevents mismatching between applied voltage and the optimum UV light capacity, such as occurs when operating with alternating voltage or alternating current, since the operating point corresponding to an optimum light yield is only cycled through briefly as the voltage changes in time.
The direct voltage operation with polarity switching at intervals known from gas discharge lamps for lighting purposes would require repeated preheating of the electrodes after each change of polarity in the case of low-pressure UV irradiation lamps. The act of preheating every 15 to 30 minutes would itself be sufficient to reduce the operating life severely. Since considerably higher radiation output is produced by low-pressure UV irradiation lamps for disinfecting water by ultraviolet light than by gas discharge lamps for lighting, and power consumption is accordingly significantly higher, the effects of electrophoresis would also become evident considerably sooner. In order to avoid the disadvantageous effects of electrophoresis, the polarity would have to be reversed at shorter intervals, and this again would drastically shorten the operating life due to the need for repeated preheating or the power load on the cooled electrodes in the case of insufficient preheating.
The dilemma described in the foregoing is resolved in the first instance by the further measure according to the invention of setting the intervals for polarity reversal to a time shorter than the time required to reach a lower threshold value for the operating temperature of the electrodes, as determined by the thermal time constant of the low-pressure UV irradiation lamp. If this determination rule is observed, the cooling electrode in each case is still at its operating temperature at the time of polarity reversal and after the polarity reversal can then assume the function of the electrode previously kept at the operating temperature without repeated preheating or wear due to excessive power loading. In this way, the advantages of direct current operation are exploited and at the same time the effects of electrophoresis and electrode wear as a result of overfrequent preheating or power loading of the electrode that has already cooled to below the operating temperature are avoided.
The switching of the polarity does not constitute conventional alternating current operation, because the switching frequency per unit of time is smaller than the lowest frequency that was formerly in common use with alternating current operation, the mains alternating current of 50 to 60 Hz. The polarity reversal also does not correspond to the zero crossover of the harmonic, particularly sinusoidal oscillation of the mains alternating current, but rather to the polarity reversal that takes place during the switching transition period, the voltage of which has at least the value of the arc drop voltage. Otherwise the low-pressure UV irradiation lamp would go out considerably before the polarity reversal, because some time would still elapse after the applied voltage dropped below the arc drop voltage value and before it finally reached the zero value.
The time intervals between polarities changes can be set to longer than 0.2 seconds but shorter than 5 seconds.
Thus, the intervals between polarity reversals are considerably longer than the period of the normal mains frequency, so difficulties from perturbing radiation will not arise and there is no risk of contravening electromagnetic compatibility regulations.
At the same time, the intervals are also shorter than the time it takes for the electrode to become cooler than the operating temperature. The thermal time constant of the low-pressure UV irradiation lamp indicated for this purpose is calculated on the basis of the combined thermal time constants for the electrodes, the gas-phase contents of the lamp, and the lamp housing and may vary from lamp to lamp. It is therefore not possible to specify an exact threshold value. It is also possible to provide for cooling below the operating temperature at the expense of the operating life of the low-pressure UV irradiation lamp. To compensate for this, a higher voltage must be applied, but this may still be below the initiation voltage. However, the greater the value by which the operating voltage is undersupplied, the greater is the power loading on the electrode, since material is torn from the surface of the affected electrode each time the polarity is reversed, and this shortens the operating life of the electrode.
The lamp voltage or the lamp current can also be monitored after a polarity change and if the electrical power deviates from a reference value, the polarity can be reversed again.
This provision ensure that the low-pressure UV irradiation lamp never operates for too long without a polarity reversal and thus avoids possible damage from the effects of electrophoresis.
The threshold value is preferably set 3% lower than the output value at the beginning of a polarity reversal.
This value, which is about 10% of the variable value assuming constant voltage and variable current or constant current and variable voltage, does not cause an apparent loss of UV output. With regard to electrophoretic effects, this is then also an initiating stage, which is still reversible after immediate polarity change, so that there is not deleterious effect on the operating life.
It is practical to establish monitoring intervals for measuring output that are shorter than the thermal time constant for the low-pressure UV irradiation lamp.
This ensures that electrophoresis effects are still detected even if they occur before the polarity reversal, the timing of which is determined on the basis of the thermal time constant of the low-pressure UV irradiation lamp.
The transition time, during which the polarity is reversed, can be set to be shorter than the recombination time for the gas discharge arc of the low-pressure UV irradiation lamp.
This provision ensures that during the transition from one polarity to the other and the change from negative to positive or from positive to negative of the value of the steady state direct voltage, the gas discharge arc is not extinguished by recombination of the gas ions that form it, so that it needs to be restruck. If an appropriately short time is set, the ionization of the gas discharge arc is not lost, so that may be retained without repeated ignition and can continue to be used to generate UV light.
The operating procedure according to the invention described with reference to the process and the advantages of the improvements apply also to the ballast. In a further improvement of the ballast, the switch is configured from four static switches in a ring arrangement that are powered with direct voltage or direct current at two opposing nodes. A bridge arm includes the low-pressure UV irradiation lamp. Two diagonally opposed static switches are opened and closed in alternating sequence with the other two opposing static switches.
This provides for steady state operation between the switching phases, and also means that the switching time when the polarity is reversed is very short.
In a further enhancement, at least one closable static switch in each pair may take the form of a controllable source of electric power.
This configuration has the advantage that a direct voltage source that is exclusively voltage-controlled can be used as the power source for the entire arrangement. The arc drop voltage of the lamp can be set here. The controllable or adjustable power sources that are present in each active branch of the circuit serve to compensate for lamp tolerances and environmentally-conditioned variations in, the electrical operating parameters of the low-pressure UV irradiation lamp.
In a further embodiment of the ballast according to the invention, the initiation device includes a series connection consisting of an inductor and a capacitor that is disposed between the electrodes of the low-pressure UV irradiation lamp. Prior to initiation, this serial connection may be connected to an alternating voltage or alternating current energy source such that it may be disconnected therefrom for initiation.
In this embodiment, the voltage source does not have to provide the initiation voltage, and can be in the normal range for the arc drop voltage. The initiation voltage is generated when the current flowing in the inductor in the series connection initially cannot continue to flow in a closed electrical circuit when the static switches are opened, which leads to a voltage buildup, and this in turn provides the initiation voltage via the parallel connection to the discharge area of the low-pressure UV irradiation lamp. After initiation, the system switches to the steady state, in which each pair of diagonally opposed static switches in the ring arrangement is alternately closed or opened to complete the connection between the low-pressure UV irradiation lamp and the voltage or current source.
The serial connection consisting of an inductor and a capacitor can also be arranged in series as a heating coil for the electrodes of the low-pressure UV irradiation lamp, and in this arrangement the alternating current applied prior to initiation is used at the same time to preheat the heating coil.
Heating coils of this nature are quite essential for amalgam-doped low-pressure UV irradiation lamps, since without them initiation cannot take place. The improvement means that the electrical circuit can be used in alternating voltage mode to heat the heating coilxe2x80x94with current limiting assured by the inductor and the capacitorxe2x80x94and to initiate the low-pressure UV irradiation lamp by means of the inductor.
An alternative embodiment of the initiation device may include a capacitor that is arranged between the electrodes of the low-pressure UV irradiation lamp. A direct voltage rising to the initiation voltage is applied to the electrodes before initiation. After initiation, and when the voltage has fallen to the arc drop voltage level, a filter capacitor is switched on by a static switch.
By rectifying the low-frequency alternating voltage from the mains supply to provide direct voltage, the filter capacitor then serves to attenuate a pulsing direct voltage component. The filter capacitor, which is larger than the initiation capacitor because of its rating for the low capacitance frequency, may be selected to have lower electric strength than the initiation capacitor because it can be switched off. The initiation capacitor is constantly parallel to the low-pressure UV irradiation lamp and must be rated for the initiation voltage.
In a serial connection of multiple low-pressure UV irradiation lamps, the initiation device can additionally include several capacitors in series that for their part are each arranged in parallel with the low-pressure UV irradiation lamps. At the same time, an embodiment of the capacitive voltage distributor may be provided with the same or different capacitors.
If the same capacitors, and consequently the same distribution ratio, are used, the initiation voltage that can be applied to the serial connection of low-pressure UV irradiation lamps and parallel capacitors at least reaches a value corresponding to the initiation voltage of the most easily initiated low-pressure UV irradiation lamp multiplied by the number of low-pressure UV irradiation lamps connected in series.
Then, when a low-pressure UV irradiation lamp has been initiated, its voltage drops to the lower arc drop voltage, so that the applied voltage is then distributed at an accordingly higher level among the low-pressure UV irradiation lamps that have not yet been initiated. These low-pressure UV irradiation lamps are then initiated almost simultaneously, for as each subsequent low-pressure UV irradiation lamp is initiated the voltage applied to the remaining low-pressure UV irradiation lamps is increased, so that even the most reluctant low-pressure UV irradiation lamps, which require a higher initiation voltage than their counterparts, are forced to initiate rapidly.
In an embodiment with dissimilar capacitors or in which one capacitor is actually missing, resulting in an unbalanced distribution ratio, the maximum initiation voltage can be limited to a value that only marginally exceeds the necessary initiation voltage for a single low-pressure UV irradiation lamp. The major portion of the initiation voltage on the serial connection is applied in the first instance only to the first low-pressure UV irradiation lamp, which is initiated accordingly.
The initiation voltage, less the arc drop voltage for the initiated lamp is distributed in the distribution ratio of the remaining capacitive voltage distributor to the remaining low-pressure UV irradiation lamps, of which one more receives a major proportion of the initiation voltage, and is initiated. This procedure is repeated in like manner until all the low-pressure UV irradiation lamps are initiated.
The supply voltage for the ballast may be variable, and in the case of multiple low-pressure UV irradiation lamps connected in series, may be adjusted for the sum of individual voltages for the low-pressure UV irradiation lamps.
This solution enables not just one low-pressure UV irradiation lamp, but serial connections of various numbers of low-pressure UV irradiation lamps to be driven by one ballast without the need for any changes to the ballast. Indeed the economic viability of the ballast is significantly increased if several low-pressure UV irradiation lamps are driven by the same ballast.